The present invention relates to the production of variable density metal vapors, including magnesium vapor, that can be used to form uniform powders of metal oxides. More specifically, the invention relates to a method for the production of metal vapors formed from the decomposition of metal carbides which bypass the liquidus phase.
Powders formed from metal oxides are widely used in a variety of industries as ceramic materials, catalysts, pigments, coatings and fillers. Some metal oxides, such as magnesium oxide and calcium oxide, have excellent heat resistance and electrical insulating properties and are especially useful in the electronics industry.
There are several standard ways to produce metal oxide powders. The most obvious approach is direct elemental synthesis. For example, MgO can be formed either by burning metal Mg ribbon or entraining Mg vapor in a carrier gas stream. The formula for this reaction is: EQU 2Mg(vapor)+O.sub.2 (vapor).fwdarw.2MgO(solid) (1)
While this method is simple in principle, it can be difficult in practice since it requires processing at high temperature which is not easily controlled. Also, this method can leave unreacted Mg in the final product and it is difficult to control the particle size of the metal oxide that is formed by such a method.
Another commonly used method for forming a metal oxide is the thermal decomposition of either the hydroxide or carbonate form of the metal. For example, MgO can be synthesized via the thermal decomposition of either magnesium hydroxide, magnesium carbonate or magnesium formate. For the hydroxide form, the formula for the reaction process is: EQU Mg(OH)2.fwdarw.MgO+H.sub.2 O (2)
This approach has the advantage of being simple, but irregular surface morphology, particle size distributions and surfaces terminated with hydroxyl groups are serious draw-backs.
It has been found that the particle size of metal oxide powders effects their characteristics and that very fine metal oxide powders exhibit unique properties that are not found in larger size particles. In particular, very fine metal oxide particles exhibit different magnetic and optical properties from those of larger metal oxide particles. This size dependant variation of physical properties is often desirable and so there is a need in the industry for a process that can produce small-size metal oxide particles.
It is known that fine metal oxide particles can be produced by various methods including liquid phase reaction methods and gas phase reaction methods. The gas phase reaction methods have been found to offer significant advantages because the oxidation of the metal vapors allows the particle size of the metal oxides to be controlled. The present invention provides a method for producing high purity metal vapors that are useful in forming metal oxide powders.